I have an old ipod (listening to it right now on the bus). I *often* sing while waiting for a bus if no one is around, but will stop if someone comes up, even though I can sing quite well. (It just always sounds weird -- like, disembodied-- when I hear people do it, so I don't.) The best is on a loud busy road with no one around, I can sing as loud as I want.
It's very hard to resist singing to certain songs especially.

So yeah, absolutely. I think it's a music-lover thing.

I also talk aloud to myself a *lot*.